Titanium-containing metals are of great interest to the aerospace industry because they have low densities, low thermal expansion coefficients, and high structural strengths. Parts made from titanium-containing metals are lightweight, and can withstand high thermal stresses and high physical loads.
In some applications, it is desirable to deposit a metallic coating onto the surface of the part. However, the part rapidly oxidizes when exposed to oxygen to create an oxide layer that is electrically and chemically passive in nature. The presence of this passive oxide layer severely inhibits the chemical bonding that takes place between the metallic coating and the part. As a result, it is extremely difficult to deposit an adherent metal coating onto the part. Even when the metallic coating is successfully deposited onto the oxide layer of the part, adhesion tends to be poor. Consequently, the metallic coating is of little value since it can be removed from the surface of the part by bending, peeling and/or scratching.
Aggressive pretreatments including the use of harsh etchants (i.e., high concentrations of chromic acid, nitric acid, sulfuric acid, glacial acetic or any combinations thereof) are commonly used to remove the passive oxide layer from the surface of the part prior to depositing the metallic coating. However, these harsh etchants cannot be used on precision electronic aerospace parts because these parts have tight tolerances and prescribed surface finishes. Moreover, the harsh etchants are harmful to humans and to the environment.